Carbonated water has been used in a bathing place using hot spring since the carbonated water has an excellent effect of keeping warm. The effect of keeping warm of carbonated water is considered to be because basically the human body condition is improved by distal blood vessel expansion effect by its contained carbon dioxide gas. Additionally, the blood capillaries increase and expand due to invasion of carbon dioxide gas into the skin, so that blood circulation in the skin structure is improved. It has been said that carbonated water is effective for diagnoses for regressive disease and distal circulation trouble.
To obtain such carbonated water artificially, there are available a chemical method of reacting carbonate with acid, a method of using combustion gas from a boiler or an apparatus which injects carbon dioxide gas directly into a pipe having a diaphragm as described in, for example, Japanese Patent Laid-Open Publication No. 5-238928 and the like. Recently, a number of methods for producing carbonated water using a membrane have been proposed. Because using the membrane enables carbon dioxide to be supplied in a very fine condition, carbonated water can be produced effectively. For example, Japanese Patent Publication No. 2810694 has proposed a method which uses a hollow fiber membrane module accommodating plural hollow fiber membranes, both ends of which are open, and as this hollow fiber membrane, porous hollow fiber membrane is employed. Further, methods of using non-porous hollow fiber membrane as the hollow fiber membrane have been proposed through, for example, Japanese Patent Publication No. 3048499, Japanese Patent Publication No. 3048501, and Japanese Patent Laid-Open Publication No. 2001-293344 and the like.
As the method for producing carbonated water using the membrane there are available what is called single-pass type of producing carbonated water by passing raw water through a carbon dioxide gas dissolver having a membrane module by one time and what is called circulation type of circulating hot water in a bath with a circulation pump through a carbon dioxide gas dissolver.
Although the single-pass type is capable of producing carbonated water in a short time because carbon dioxide gas is dissolved in water all at once, it has such a disadvantage that its dissolution efficiency is lower than the circulation type so that a high concentration is difficult to obtain. Thus, it has been demanded to improve the dissolution efficiency further. On the other hand, although the circulation type ensures a higher diffusion efficiency of carbon dioxide gas than the single-pass type so that a high concentration of it is easier to obtain because it is dissolved in water slowly, it takes long to dissolve until a predetermined concentration is reached and thus, it has been also demanded to improve the dissolution efficiency further.
Accordingly, an object of the present invention is to provide an apparatus for producing carbonated water and a method for producing carbonated water using the same apparatus, capable of obtaining carbonated water having a high dissolution efficiency of carbon dioxide gas, that is, a high concentration of it in a short time easily.